Polymeric protective foams (e.g. protective foam layers) are widely used for impact force attenuation in a variety of safety-related applications. These include but are not limited to automotive applications, sport applications, bedding applications, footwear applications, and etc. In general, a protective foam layer is placed adjacent or against a part of a person's body in order to protect that body part (e.g. a head) during an impact with, for example, the ground or even another person's head.
Protective foams function by absorbing and/or dissipating the impact energy from the force of an impact. An energy absorbing foam deforms or crushes upon impact thereby consuming a portion of the impact energy so that portion does not reach the underlying body part. An energy dissipating foam also spreads the impact force over a larger surface area than the actual area of impact so that the force per unit area is decreased for the underlying body part compared to that for the initial impact surface (e.g. the outer surface of the protective layer or a hard outer shell over the protective layer).
All rigid or semi-rigid protective foams are energy dissipating foams to some extent because, due to their rigidity, they do not instantaneously yield upon impact. Instantaneous yielding would result in the transmission of the entire impact force to the localized region of the underlying body part immediately beneath the protective layer at the point of impact. Instead, rigid and semi-rigid foam layers typically have sufficient rigidity to transmit at least a portion of the impact energy from the point source (impact site) to lateral or adjacent regions of the foam layer before the energy is transmitted to the underlying body part. The result is to spread the impact force over a larger area and thereby reduce the force per unit area experienced by the underlying body part as described above.
However, traditional rigid and semi-rigid foams exhibit satisfactory energy absorption only above certain impact speeds, e.g. above about 4–7 meters/second (m/s) for expanded polystyrene (EPS) which is the most common rigid foam found in bicycle and motorcycle helmets. This is because the foam is so stiff that it must experience a minimum threshold impact velocity in order for there to be sufficient energy to crush the foam Practically, this means that up to this threshold velocity, virtually all impact energy will be transmitted to the underlying body part and not absorbed by a rigid foam like EPS. An additional problem with EPS foams is that they are non-recovering; i.e. they do not recover or rebound to any significant degree once they have been crushed from an impact. They are effective for only single-impact use and then must be discarded.
Existing semi-rigid polyurethane foams address these shortcomings to some extent as a result of their limited viscoelastic properties. Though existing semi-rigid foams can be compressed or deflected at lower impact velocities to absorb some degree of the impact energy, they cannot effectively absorb the energy from higher velocity impacts compared to rigid foams like EPS. Some designers have attempted to formulate urethane foams that are more rigid and can provide protection similar to EPS. However, these more rigid urethane foams also correspondingly begin to suffer from the same drawbacks, which initially led the designers away from rigid foams like EPS. The more rigid the foam, the less it will recover after being crushed, and the poorer low to moderate impact energy absorption it will provide.
Consequently, there is a need in the art for a semi-rigid viscoelastic polymeric foam that is rigid enough to provide adequate impact energy absorption at high impact speeds, e.g. 4–7 m/s or greater, and yet recovers substantially 100% after impact. Most preferably, such an improved foam will also provide adequate low to moderate speed impact protection to the underlying body part of a user of the foam.